I'm replacing a thermostat and of course there is an exra wire. In my old stat (honeywell chronotherm III) there is a terminal marked X. There is no information in the installation guide for new t stat(rth7400)for this terminal. It goes to the air handler and then to the heat pump where it is connected to a white wire. Model York E1RA030S06D.What does it do so I can connect my new t stat?Thank you.